Recombinant tumor necrosis factor (rTNF) inhibits erythropoiesis. To human CFU-E rTNF was added and found to have a marked inhibition, which disappeared after purification of the CFU-e. Inhibition of colony growth was restored by adding back accessory cells or conditioned medium from cells stimulated by rTNF. Indications are that the anemia of chronic disease is not a direct effect of TNF but an indirect effect.